Last night I ate a large bowl of beetroot from my garden. This morning my urine is the colour of rosé wine and I'm worried that my semen might have taken on a similar hue. The colour of my semen is a concern because someone will be studying it in a short while. I'm considering this while sitting in the top floor 'specimen room' of the London Fertility Centre on Harley Street. Later on, when I mention where I've been to friends and colleagues they seem really interested in the interior design details of a room set aside for masturbation. So if you're planning one, here's some decorating tips. The room is on the second floor and it has two notices on its door: one saying 'Quiet Please' (in case passers-by are inclined to cheer or clap, I guess) and a sliding sign with 'Vacant/Occupied' options - I've opted for 'occupied' although I'm not, so far. Inside, the room is about 6ft x 12ft and painted in various pale non-colours. It is equipped with an ensuite shower, light-green vinyl-covered daybed and a fudge-coloured bathroom suite (including bidet). There is a sash window - which isn't overlooked. The atmosphere is more Carry On than Casualty. On one side of the sink there is a small empty plastic beaker (with my name on it). On the other a DVD player, screen and a remote. I consider all the hands that have touched the remote. Using one of the many tissues provided I pick it up and inspect it; it appears to be clean. The television doesn't show any of the normal channels.

I'm here because I'm concerned about my sperm. Not that they might be beetroot coloured, but rather that they might not be fit for purpose. That they might not be as athletic, plentiful and perfectly formed as they need to be. I'm 41 and childless, and although I'm not involved in a 'trying-for-a-baby'-type scenario I've been reading the papers and the news for fortysomething men and their sperm isn't great.

'Scientists warn that biological clock affects male fertility' warned the Guardian in July - well, scientists are always saying stuff aren't they? 'Risk of miscarriage soars once the father reaches 35' (Daily Mail) - that sounds worrying. 'Blokes going infertile aged 35' (Sun). Must have sex, pronto! The papers were all reporting in their own particular ways on the research of Dr Stephanie Belloc from the Eylau Centre for Assisted Reproduction in Paris. Dr Belloc had studied the records of 12,000 couples who visited her clinic and separated out the influence of the mother's and father's ages on the chances of conception and miscarriage.

Belloc and her team found that women whose partners were 35 or older had more miscarriages than those who were with younger men, regardless of their own age. The risk of miscarriage was on average 16.7 per cent when the men were aged 30-34, but it doubled to 33 per cent in men over 40. Moreover, her research showed that men's ages also affected pregnancy rates, which were lower in the over-40s. As the Mirror summed it up, 'Over-35? You're a dad loss.'

I can remember ridiculing my own father for being 40, so how did I end up childless at 41? To start with I went to university and became middle-class. It seems only people from council estates and people who own estates have kids young these days. The middle classes are too busy in their twenties establishing careers, climbing the property ladder and going on snowboarding holidays.

Although lack of one doesn't stop some people, I feel you need to be in a reasonably stable relationship before having kids - and I haven't been in one of those of late. But of late, many of my peers are reproducing, some are already on to their third. Even the ones who had drug problems are conceiving and, meanwhile, gay friends are cutting breeding deals with lesbians. I wonder if time is running out.

It's an easy thought to have because I can't act on it, but sometimes I think I should have had some children in my twenties. I had more energy and didn't have many material comforts to give up or much of a lifestyle to compromise. I'd be packing them off to university around now, thumbing sports car brochures and thinking about buying a peach farm in Spain. Frankly, I can't remember that much of my twenties, so maybe it would have put this decade of void to good use. I don't recall any of my peers having kids; maybe it was a hangover from the Aids era - people seemed pretty conscientious about birth control, there were no 'accidents'. So now, at 41, I wonder if I've skipped the whole kids thing.

I seem to be developing the hobbies and pastimes of a senior citizen - golf, growing beetroot, buffing my classic car. But the reality is I've got 19 years until I qualify for my bus pass - which is just enough time to raise at least one human being. So should I be worried about or believe in the 'male biological clock'?

Back in 2001, Professor Dolores Malaspina, of Columbia University College of Physicians and Surgeons, concluded that men aged 50 or over are three times more likely to father a child with schizophrenia compared with men of 25 or under. Four years later, epidemiologist Jorn Olsen at the University of California, Los Angeles, found a fourfold rise in Down's syndrome among babies born to men aged 50 and older. And in 2006 scientists from the Institute of Psychiatry at King's College, London and Mount Sinai School of Medicine in New York found that children born to fathers aged 40 and over were nearly six times more likely to suffer from autism than those with a father under 30. Meanwhile, other researchers have suggested patterns between older fathers and increased chances of bipolar disorder, dwarfism and Apert syndrome - whose unlucky sufferers have a malformed skull and webbed hands and feet, among other disfigurements. A report in 2006 even suggested 'a modest effect of advanced paternal age on the Apgar score'. And after finding out what an Apgar score is I now know this to be less than good. The evidence appeared to be stacking up.

Yet are these findings as scary as they sound? Dr Belloc's sample was made up entirely of couples presenting for infertility treatment. 'It is not evident that we can extrapolate these conclusions to a fertile population,' she tells me. And many of the incidences in the other studies are minute; so a fivefold increase is still only a five-times-minute chance of some disorder or other. Moreover, these studies only show patterns, rather than direct causal links - finding a direct link would probably require examining DNA at a detail beyond most researchers' budgets or ability. Some commentators have speculated that if a man first becomes a father in his forties or fifties that may indicate he has had trouble forming relationships earlier in his life, which may mean in a mild, undiagnosed kind of way he's a carrier of problems like bipolar disorder or autism which have a genetic element - so his paternal age is irrelevant to the outcome.

Which isn't exactly comforting, but it suggests the 'male biological clock' doesn't tick as loudly as the headlines suggest. For Dr Allan Pacey, senior lecturer in andrology at Sheffield University, the clock is nothing more than ageing. As you grow older, you lose a bit of hair and experience the odd 'senior moment', so you shouldn't be surprised if your sperm isn't as sprightly as it used to be. 'In terms of numbers it's the same, but what tends to happen is that the sperm isn't as good.' If their biological clock is ticking, men are pretty deaf to it. The age of fatherhood is creeping up: the latest figures from the Office of National Statistics show that the average age of married fathers rose from 29.1 in 1971 to 34.1 in 2003 - getting close to the 35-year point where some of the problems are alleged to kick in. I ask Dr Pacey if this is a worrying trend. 'The problem is couples are waiting until they are older. To wait until the woman is approaching 40 is the wrong time to be starting, and that will be exasperated by any problem that he has due to ageing.' Dr Pacey's advice to me is not to hang about: 'You will be more successful having a child naturally at an earlier age; it will be cheaper for you and it will be much more fun than waiting until you're well into your forties, going to an infertility clinic and having it done artificially. What we're finding are lots of people attending infertility clinics in their forties who would have succeeded in getting pregnant at 25. Rather than waiting for technology to sort it out, if you are in a position to have children early, then go ahead and do it.'

What Dr Pacey and others are quick to point out is that there's definitely a female biological clock. Women are born with a finite number of eggs and at some point they will run out. According to the Human Fertilisation and Embryology Authority (HFEA), a woman is half as fertile at 35 as she is at 25, and half as fertile again at 40.

You might be thinking, 'Why is he bothering to spell that out, everyone knows that?' Well, before researching this piece I was only vaguely aware of those blunt facts, but, more surprisingly, when chatting to single and married thirtysomething childless women about this article they start saying things like: 'My gran had my mother at 45,' 'What about Madonna?' or, most biologically incorrect: 'I'm not ready yet.' They seemed about as informed as I was. 'With the Madonnas and all the rest who seem to have children quite naturally, no one mentions IVF or egg donors, and celebrity miscarriages don't make the pages of Heat,' says Dr Pacey. 'This silence reinforces the myth that these miracle births happen, when often there's a medical intervention.' And IVF isn't a safety net: according to the HFEA, IVF has only a 12 per cent success rate for a 40-year-old woman. And it will cost you: the NHS, on the advice of the National Institute of Clinical Excellence (Nice), doesn't fund IVF for women over 40 because of the low success rate. The average cost of a cycle is £4,000-£8,000. Is it chauvinistic to question the sense of delaying having kids for the sake of a career if you're going to spend most of the extra income on fertility treatment?

However it's not only career building that is nudging the maternal age up; those commitment-phobic, nappy-changing-averse partners make a contribution, too - people like me. One could argue that this male biological clock business is providing men with another excuse to avoid having kids - we move from 'I'm not ready yet' to 'It's too dangerous now' in the time it takes to power up a Nintendo Wii. Or maybe you could blame the introduction of Viagra - which has engendered the idea that men can stay virile forever, so why rush? - as most men think the difference between virility and fertility is latex thin. But if you're looking for something that's really obscuring the hands of the male biological clock, look to famous people. When it comes to fertility, biology tells us one thing, but celebrities tell us another: ie, no matter how superannuated you are, getting your girlfriend up the duff is child's play. Middle-aged famous fellas love a baby shower.

Dr Pacey isn't impressed: 'The John Humphrys thing does distort the picture. There'll be lots of men who will read this piece and say, "I was 50 and I had a child," and it's really difficult to argue against that because they do, but statistically you are less likely to succeed and more likely to have problems. For the individual who has been successful it will seem stupid that I'm saying that, but for every 50-year-old father there'll be 10 times more thinking, "I had a lot of problems."'

Even if you, your sperm and your wife from a younger generation manage to buck the stats, there are other non-bio reasons against fathering kids late. Most obviously you might die before they graduate - if you're 65 now, on average you'll die at 82 - although for how much longer you will be capable of having a kick-about, helping them with their homework or visiting the lavatory without their assistance isn't recorded. And while it's embarrassing to be mistaken occasionally for their grandfather, it's thoughtless not to meet your grandchildren.

Am I being too hard on the older dad? I call Charlie Lewis, professor of family and developmental psychology at Lancaster University. Should we give middle-aged men the snip? 'Some men claim to be better fathers when older, but I don't see this in the majority of men. I find them saying, "I'm clapped out, I've done my bit at work, I've provided a house and comfortable living, now let me vegetate." They think it's their right to sit in front of the telly and not take part in any interaction. It's almost autistic. Older fathers tend to do less of the stereotypical activities than younger fathers do, less childcare and less kicking footballs - for fear of snapping a tendon. They think, "I'm much too old for this."'

Surprisingly, Lewis is more relaxed about the dying thing. 'I don't want to put fathers down, but if you look at the majority of evidence on loss, it does point to losing a mother before 11 being more predictive of later social/psycho disorders than losing a father. These effects are most often caused by the child absorbing the surviving partner's grief. So if the mother can manage the grieving process, the predictable death of an older father needn't be a life-changing trauma.'

Dads dead or alive, we should be more concerned about the kids, says Lewis. 'You do get studies that say old dads feel closer to their kids, but I'm not aware that kids feel closer to their older fathers.'

I wonder if I would become one of these dead-beat, distant dads. I like to think not. I don't quite understand how

that could happen. What kind of an individual would tune into a Top Gear repeat rather than read to their child or even relieve them of a shitty nappy? Maybe I'm being naive. I talk to some dad friends.

Gary, 45, first became a father when he was 23, but then remarried and had three more children, the oldest of whom is five. Would he like to compare and contrast? 'Obviously becoming a father young was a bit of a shock, it made me grow up quickly. I'm not sure at that age if you're responsible enough to look after yourself let alone a little child.' So how is it second time around: does older dad mean better dad? 'When my second wife first wanted children I did have slight panic attacks, because I had this memory of it being a total whirlwind, but this time it's completely different, it doesn't seem half as stressful as when I was in my twenties.' Gary says this isn't just because he's been a parent before - 'No, it's mainly because I'm more grown-up, more patient, more financially settled. I'm far more chilled out this time around.' So you'd advise an older option? 'It's better to have children at a later date, but myself, I'm worried about getting older. First time round I was one of the youngest parents in the playground; now I'm one of the oldest. My youngest is 10 months, so I'll be at retirement or grandfather age in her late teens. You hope to be running around in the park, doing those things that children want you to do and provide as parents. Hopefully I'll be one of those who manages it, but I will have to wait and see.'

The energy issue: I've heard this raised before. People talk about the nuclear-like amounts of energy you need to bring up a child, but I suspect it's similar to the stamina needed to squire a girlfriend half your age. Because down-ageing your just-broody girlfriends each time they start describing a new frock as 'a bit maternity' is really the only alternative to producing offspring.

Jonathan, 49, had two sons when he was 23 and 27. He says the early months were 'terrifying', and both he and his girlfriend had to abandon their career plans: 'Our embryonic lives together as a couple were entirely transformed into a fully fledged proper adult relationship. And we didn't have much money - I even used to scavenge skips for firewood.' But for all the foraging the relatively small age difference means he's closer to his kids. 'We can go to the cinema together, appreciate some of the same music, go out for a beer, they call me by my first name.' He got divorced and, a couple of years ago, he remarried. He isn't keen to become a father again: 'I'm interested in the relationship with my wife rather than with anyone else. The relationship I have with my children is established, I like the marriage and lifestyle we have, and because of my previous experience I can see how that could be compromised.'

What is his advice for someone like me, thinking of becoming a father in my forties? 'I think, you're not going to get a lot of sleep. And by the time you're my age, when you take your kids to a restaurant they'll be running around banging their heads, stealing food, whereas I'll be discussing the amount of oak in the Sauvignon with mine. I'd think about that quite carefully.'

So that's what I should have done. Bred early. Guess there's no point in crying over spilled, er, milk.

The trouble with this when-to-procreate business is it's personal. Apologies, it's not much of an insight but everyone is different. They earn lots of money, earn not much money, like kids, don't like kids, have live-in help, are still looking for The One, are given a babies-or-else ultimatum by their partners, had a shit childhood themselves, don't feel the need to have babies to preserve their relationship, are worried they'll pass on a condition, feel they've established their career, don't want a career, haven't been to Patagonia yet - the list of caveats and factors that make it the 'right time' for someone is as long as the waiting list for a Doctor Who Dalek Electronic Voice Changer Helmet.

So, to borrow a phrase from a Dragon: 'Let me tell you where I am.' For me, I think 45 is the cut-off. For biological reasons - you can't donate sperm past 45 - there must be something in those scary reports. And financially, I'd like to retire on time, if indeed I'm lucky enough to still have a career by then. Which doesn't give me much time, I guess, to meet someone, fall in love, imagine being with this person for the foreseeable future - if that's not over-romantic, delusional, too-much-like-a-John-Cusack-movie. But I'm getting ahead of myself: maybe I'm firing blanks anyhow.

For the 20-minute wait while my sperm is being tested, I chat to Dr Magdy Asaad, clinical director, in his office about the problems with semen. Mine is being tested for volume, viscosity, concentration, mobility, morphology and antibodies.

Dr Asaad uses the gold standard WHO criteria which are surprisingly generous - only 50 per cent of your sperm needs to move, for instance, and you're allowed up to 80 per cent with an abnormal form, such as funny-shaped heads or two tails, 'because 20 per cent of 20m is considered enough, it's a lot of sperm,' Dr Asaad chuckles.

I'm curious: do anxious men often pop in on their own for a lunchtime sperm test, check everything is wriggling right? 'It's not common, but when men present on their own, it's normally a problem with their ability to have an erection or ejaculation.'

Well as you can tell I have no problems in that area, I say.

'But some men don't like to give a sample,' he continues. 'They find all kinds of excuses: maybe they are worried it will not be good, or that it's an artificial thing, to press a button [is he talking about the remote control?]. I don't know how it was for you, I'm not asking. Sometimes a gentleman will have difficulty preparing manually.' Unbelievable.

The walls and desk of the doctor's office are smothered with framed photographs of beaming parents with their children - patients he's helped to fashion a bundle of joy for over the years. In your experience, I ask Dr Asaad, when is a good age for procreation? 'You're mature enough by your late twenties, early thirties, responsible enough, you probably have a job, a partner. I don't think it's a very serious problem waiting to 40-45, but beyond that you have to think about time with the child.'

With that, Dr Asaad prints off a piece of A4 containing all my sperm's vital statistics. 'It's a good sample,' he says, 'so you're all right.' I'll spare you the details.

On one hand this is a relief, but on the other it means I've no alibi, no excuses, I'm ready to breed. All I need now is a woman.

Paternity frights: ten bus-pass fathers

Julio Iglesias Sr, a dad at 89

Nobody could accuse the gynaecologist father of Julio and grandfather of Enrique, and who was head of a Madrid family-planning unit, of not taking his work home with him. After having two children with his first wife, he remarried and, at 89, when his wife was 40, produced another son. Barely out of the maternity ward, Ronna signed up for IVF and within a few months was pregnant again. Tragically, filling a test-tube turned out to be the former Franco supporter's last significant act: two months later he was muerto. His daughter Ruth was born posthumously seven months later in July 2006.

Dad-speak: 'At my age, a child is marvellous. I felt just like Abraham. It was an act of generosity towards her [Ronna]. I leave her part of my blood, of my life.'

Saul Bellow, a dad at 84

The Nobel Prize-winning novelist had four children: three sons with his first three wives, and a daughter, Naomi-Rose, with his 41-year-old fifth wife. He died when she was five, in 2005. Writing two months after his death, one of his sons, Adam, whose mother Bellow left when he was two, recalled 'a fond but highly attenuated bond with a frequently distracted, often absent and much older father.'

Dad-speak: 'Well, my wife won't be lonely when I die. She'll have somebody'

Anthony Quinn, a dad at 81

The star of more than 100 movies, including Zorba the Greek and The Guns of Navarone, enjoyed procreating. He had five children with his first wife Katherine, the daughter of Cecil B DeMille, three with the second, then at the age of 81, he got his 29-year-old secretary pregnant, married her and had two children. The double Oscar-winner also squeezed in three more children with women he wasn't married to before he died in 2001.

The Australian-American global media mogul (real first name Keith) has been married three times. He produced one child with the first and three (Elizabeth, James and Lachlan) during a 31-year marriage to the second. Seventeen days after the $1.2bn divorce, the Dirty Digger married former photographic model Deng Wendi (she transposed her names post nuptials), a 30-year-old executive at his Asian Star TV channel. They have two children, the most recent in July 2003.

Dad-speak: 'All my children will be treated equally'

Des O'Connor, a dad at 72

The former Countdown host has been married four times and has four grown-up daughters. His current wife, the 37-years-younger singer/dancer Jodie, who he met in 1990, when they were doing panto together, provided him with a son in September 2004.

Dad-speak: 'When the baby was born the odd comment was made about my age, but I plan to play football with Adam'

Luciano Pavarotti, a dad at 67

The well-upholstered tenor had three daughters with his first wife, who he stayed with for 35 years. Then, in 1996, he left her for his secretary, Nicoletta - 36 years his junior. In 2003 she gave birth to twins, another daughter and a son; tragically, the latter was stillborn. 'The King of the High Cs' died after a long battle with pancreatic cancer just before his youngest daughter's fifth birthday.

Dad-speak: 'I never imagined that at this time of life I would have another child. But I met Nicoletta, and she is young'

Warren Beatty, a dad at 62

After years of womanising (Natalie Wood, Julie Christie, Isabelle Adjani, Vivien Leigh, Cher, Madonna, Carly Simon, Barbra Streisand, Britt Ekland, Diane Keaton, Mary Tyler Moore, Janice Dickinson and Faye Dunaway to name a few) he plumped for Annette Bening. They've had four kids, the latest of whom was born in 2000. I think we can assume fatherhood has mellowed Warren.

Dad-speak: 'We're fortunate to have a big house'

Rod Stewart, a dad at 60

The rooster-haired senior citizen has been breeding for 41 years. He's had seven children by five different women, although modest Rod often downgrades to six offspring, passing over his first, who was put up for adoption: 'You can count her if you want. I try not to,' he once said. Penny Lancaster provided him with his sixth/seventh, Alastair, in 2005. According to his brother Don, Rod prefers to leave Alastair's nappy-changing and feeding to the hired help. Unperturbed, 37-year-old Penny has dropped heavy hints she'd like a second with the 63-year-old Celtic fan.

Dad-speak: 'I didn't see my oldest kids a lot as they were growing up. I don't feel any guilt, but maybe having a family is something Rachel and Alana and I should have thought about more before we had children'

Michael Douglas, a dad at 58

The Basic Instinct star had a son, Cameron, with Diandra Luker, his wife of 23 years. She divorced him in 2000. Later that year he ran into Catherine Zeta Jones and seduced her with the admirably direct and honest line: 'I'd like to father your children.' True to his word he hasn't let the 25-year age gap stop him from impregnating her twice, when he was 55 and 58.

Dad-speak: 'It's not that I didn't enjoy it the first time, but I just didn't have the time. I'm not the only father who has felt guilty about the lack of time spent with his kids. So now I have a situation where I can savour it with my younger children. And you can see the effect of hanging out with them for three years and the security they have. And for me, it's a ball. Movie roles come and go and it's a finite period of time. This is sort of eternal'

John Humphrys, a dad at 56

The Welsh son of a hairdresser and French polisher has been married twice. The first wife provided the Mastermind host with two children, now both grown up. He remarried in 1987 and, after a reverse vasectomy, the Today programme interrogator became a proud father to a son, Owen.

Dad speak: 'I thought I might resent this little kid for buggering up my life, as it were. The opposite has happened to me because of him. He's the most wonderful thing that's ever happened to me'